Process for making nano-sized and sub-micron-sized lithium-transition metal oxides

ABSTRACT

A process is provided for making nano-sized or sub-micron sized oxides of lithium and a transition metal. The desired size is generally in the range 10 nm to 1000 nm and preferably in the range 10 nm to 100 nm. The particles have a narrow size distribution. The process includes milling and re-firing under controlled conditions so that crystallites of the desired particle size are grown.

The present application claims priority to U.S. Ser. No. 60/362,723filed Mar. 8, 2002, the entire contents of which is hereby incorporatedby reference.

The present invention relates to a process for producing nano-sized andsub-micron-sized lithium titanate, lithium manganate, lithium cobaltoxide and other oxides of lithium and transition metals. It covers partsof the process and the product of the process. The starting material isa coarse oxide with low surface area. The product made according to theprocess of the present invention has a high surface area and a narrowparticle size distribution.

BACKGROUND OF THE INVENTION

Lithium-transition metal oxides are materials presently used or underdevelopment for the electrodes of lithium ion batteries. The transitionmetals Co, Mn, Ni, Ti, and V have received particular attention for thisapplication. Recently, it has become apparent that a smaller particlesize and a narrower particle size distribution are beneficial forproducing electrodes, which retain their charging capacity at highcharging and discharging rates.

A method to prepare lithium titanate from inorganic solutions orsuspensions is described in U.S. Pat. Appln. Pub. 2003/0017104 A1, therelevant portions of which are incorporated herein by reference. Thatapplication describes a process to produce lithium titanatecrystallites. The process achieves good phase and size control in therange of 5 to 2000 nm. In general, the process includes providing asource of lithium titanate with a particle size smaller than the desiredparticle size and re-firing the lithium titanate under conditions toproduce a final lithium titanate having a desired particle size with anarrow size distribution and controlled surface area.

That application describes that a source of lithium titanate is from aprocess that includes forming a blend that comprises titanium andlithium. The blend is evaporated to form homogeneous particlescontaining a lithium salt and titanium dioxide. The evaporation isconducted at a temperature above the boiling point of the solution inthe blend but below the temperature where reaction of the lithium saltand the titanium dioxide occurs. The homogeneous particles are calcinedto form lithium titanate.

The lithium titanate is milled or crushed to a size smaller than thedesired size of the final product. Finally, the milled lithium titanateis re-fired under conditions to produce lithium titanate having adesired surface area and size distribution.

The blend of titanium and lithium can be provided from a variety ofsuitable sources. For example, the blend of titanium and lithium isprovided as aqueous chloride solutions of titanium and lithium.Alternatively, the blend of titanium and lithium is provided as asuspension of amorphous titanium dioxide in a lithium solution. In thisinstance, the lithium solution can be formed from a source of lithiumselected from the group consisting of lithium chloride, lithium nitrate,lithium hydroxide, lithium sulfate, lithium oxide, lithium fluoride,lithium bromide, and mixtures thereof. In yet another alternative,lithium titanate, made by any known means and having a particle sizesmaller than the particle size of the desired product, can be used asthe source of lithium and titanium for the re-firing step, wherecrystals are grown to the desired size.

A method to prepare mixed metal oxides and metal oxide compounds is alsodescribed in U.S. Pat. Appln. Publication U.S. 2002/0071806 A1 therelevant portions of which are incorporated herein by reference. Thismethod applies to mixed oxides of lithium and transition metals.Products made according to this patent application can be used asstarting materials for the process of the present invention.

Materials commercially available for the manufacture of batteryelectrodes generally have a wide particle size distribution and includelarge particles of several microns in size as well as very fine dust.Therefore, there is a need for materials having a narrow sizedistribution and having a controlled surface area for such applicationsas electrodes for batteries.

SUMMARY OF THE INVENTION

The present invention provides a process to produce lithium-transitionmetal oxides in the range 10 to 1000 nm, and preferably 10 to 100 nmwith a narrow particle size distribution. The phrase narrow particlesize distribution means that the particle size of the lithium-transitionmetal oxide is within 10 nm to 1,000 nm with a standard deviation of nomore than 20%.

In general, the process starts from a coarse oxide of lithium and atransition metal. The coarse oxide of lithium and a transition metal canbe provided by any suitable method including using commerciallyavailable coarse oxides. The process produces nano-sized crystalsthrough a combination of milling, dispersion and calcining (re-firing)steps. The transition metal may be any metal commonly defined astransition metal, including but not limited to Ti, Co, Mn, V, Fe, andNi.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet of the general aspect of the process.

FIG. 2 is a flow sheet of the process according to one embodiment of thepresent invention, where the starting material is lithium titanatespinel and the final product is a slurry containing nano-sizedparticles.

FIG. 3 is a flow sheet of the process according to the invention, wherethe starting material is lithium manganate LiMn₂O₄ or lithium cobaltoxide LiCoO₂ and the final product is a nano-sized dispersed lithiummanganate or lithium cobalt oxide powder.

FIG. 4 is a scanning electron micrograph of lithium titanate spinel ofabout 1 to 2 μm in size, serving as starting material for the process ofthe present invention.

FIG. 5 is a scanning electron micrograph showing lithium titanate spinelproducts of different particles sizes, produced following the process ofthe present invention.

FIG. 6 is a scanning electron micrograph showing commercial lithiummanganate used as starting material for the process of the presentinvention.

FIG. 7 is a scanning electron micrograph showing lithium manganateproducts of different particle sizes produced following the process ofthe present invention.

DESCRIPTION OF THE INVENTION

According to the present invention, a process for makinglithium-transition-metal oxides is provided. In this process, a lithiumtransition-metal oxide is milled or crushed to a size smaller than thedesired size of the final product. The milled or crushed lithium-metaloxide is re-fired under controlled conditions to producelithium-transition-metal oxide (e.g., lithium titanate) having a desiredsurface area and size distribution. Further processing may includedispersion, remilling, slurrying and spray drying and the final productmay be a slurry, a spry dried powder consisting of agglomerates ofnano-particles, or a fully dispersed powder.

Turning now to FIG. 1, a flow sheet according to the general process isshown. Coarse lithium-transition-metal oxide particles are milled 40 toa desired median size. After milling, the lithium-transition-metal oxide(e.g., lithium titanate) is dried 45 and re-fired in a controlledtemperature furnace 50 to produce particles having a desired size andsize distribution.

Thereafter, the particles produced from the re-firing can be dispersed60 or can be milled 90. If the particles are dispersed, they may befurther processed or may be left as is. Further processing may includeforming a lithium transition-metal oxide slurry 70, which can be furtherprocessed by spray drying 80 to produce spray-dried powder agglomeratesthat consist of primary particles. The spray-dried powder agglomeratesmay be sold or may be further processed by milling 90 to produce a fullydispersed powder.

The specific steps of the process will be explained in more detailbelow.

Starting Material

The starting material of coarse particles can be made by any method. Forexample, commercially available coarse lithium-transition-metal oxideparticles can be used as the starting material. Alternatively, as notedabove, one suitable method is described in U.S. Pat. Appln. Publication2003/0017104 A1, the relevant portions of which are incorporated hereinby reference. While the process described in U.S. Pat. Appln.Publication 2003/0017104 is directed to lithium titanate; it has nowbeen found that the described process can be used to make the lithiumtransition-metal oxides described in the present application. Inaddition, a related method is described in U.S. Pat. Appln. Publication2002/0071806, the relevant portions of which are incorporated herein byreference.

According to those processes, a blend of a transition metal and lithiumis provided by providing a source of lithium and a source of atransition metal. This blend may be referred to herein as thelithium-transition-metal blend or the transition-metal-lithium blend.

After the lithium-transition-metal blend is created, the blend isevaporated. The evaporation process is conducted above the boiling pointof the liquid in the blend and below the temperature where significantreaction of the lithium and the transition-metal compounds occurs orwhere there is significant crystallization of lithium-transition-metal.

The evaporation is conducted under conditions to achieve substantiallytotal evaporation and to form an intermediate. In particular, theevaporation is conducted at a temperature higher than the boiling pointof the blend but lower than the temperature where significant crystalgrowth of an oxide phase occurs. The evaporation may be conducted at atemperature higher than the boiling point of the blend but lower thanthe calcination temperature of the intermediate.

The term “substantially total evaporation” or “substantially completeevaporation” refers to evaporation of greater than 85% of the free watercontent, preferably greater than 90% of the free water and morepreferably greater than 95% of the free water present in the feedsolution. The term “free water” is understood and means water that isnot chemically bound and can be removed by heating at a temperaturebelow 150° C. After substantially total evaporation or substantiallycomplete evaporation, the intermediate product will have no visiblemoisture present.

The evaporation process is performed in a manner to control the physicalform of the product. Preferably, the evaporation process is accomplishedby spraying the blend while it is heated at a temperature in the rangefrom about 120° C. to about 350° C., and desirably in the range fromabout 200° C. to about 250° C. This process may be conducted in a spraydryer.

As noted in U.S. Pat. Appln. Pub. 2003/0017104 A1, the evaporationprocess may be conducted in such a manner as to form a film of a mixtureof a lithium compound and an amorphous oxidized transition-metalcompound. In this regard, the evaporation process may be conducted insuch a way as to form a thin film of lithium salt on the preexistingparticles of amorphous oxidized transition-metal compound.

In both cases, through control of the operating parameters, includingtemperature and concentration of transition-metal and lithium, thecharacteristics of the solid intermediate product can be reliablycontrolled within a fairly narrow range. For example, the productresulting from injection in a spray dryer, will generally be composed ofhollow spheres or parts of spheres. The dimensions of the spheres mayvary from less than 0.1 μm to 100 μm or more in diameter and a shellthickness in the range from about 30 nanometer to about 1000 nanometeror more. The structure of the shell consists of an intimate mixture oftransition-metal and lithium compounds.

Evaporation by spraying also has the advantage of direct processing ofthe solution so that a homogeneous intermediate product is formed and sothat evaporation of water and acid is simultaneously accomplished.Preferably, from about 90% to about 99% of any aqueous material isevaporated.

The product resulting from the evaporation step is calcined at atemperature and for a length of time sufficient to convert the mixtureof transition-metal and lithium compounds to lithium transition metaloxide of the desired structure and particle size. Calcinationtemperatures can range between about 600° C. to 950° C. Desirably, thecalcination is conducted at temperatures ranging from about 700° C. toabout 900° C. The calcination time varies over a wide range, from about1 hour to as long as 36 hours. Desirably, the calcination time is in therange from about 6 hours to about 12 hours. Lower temperatures willrequire longer calcination times. The product of calcination shows astructure of individual units that can be broken up by milling intoparticles of the desired median size and size distribution.

During calcination, the lithium salt reacts with oxygen and water in thefurnace atmosphere to release, for example, HCl gas or nitrous andnitric oxides or other gases formed by decomposition of the salt presentin the original solution. These gases may be recovered. The calcinationconditions are chosen such that contact with oxygen is sufficient tosubstantially convert the mixture to a lithium transition-metal oxidewith low impurity level.

The product of calcination may contain traces of the original lithiumsalt used as feed. To remove the traces of salt, the particles may besubject to one or several wash cycles. In each cycle, the particles aremixed with water and are separated by settling or filtration. Thewashing step is particularly useful if the lithium salt used is lithiumchloride.

Milling

The lithium-transition metal oxide is suspended in water and milled in ahorizontal or vertical pressure media mill to crush the crystals to asize smaller than the size desired in the final product.

Drying

The wet-milled particles are dried by any known means. For example,wet-milled particles may be dried in a spray drier at a temperature fromabout 120° to about 350° C., desirably from about 200° to about 250° C.Drying may also be part of the re-firing process.

Re-Firing

After milling or drying, the product is re-fired in acontrolled-temperature furnace to make a product with a well-controlledspecific surface area, consisting of regular-shaped crystals with anarrow size distribution. The refiring temperature is chosen to achievethe desired particle size and surface area of the product. In general,the re-firing temperature is between about 250° and 900° C., and the BETsurface area of the re-fired product is in the range 5 to 100 m²/g, withthe higher re-firing temperature corresponding to the lower specificsurface area.

Dispersing

After the refiring step, the product may be dispersed 60 to separate theagglomerates formed during re-firing into distinct nano-sized particles.This step is generally accomplished after slurrying the product inwater. Alternatively, the product of the re-firing step may be milled90, i.e., dry-milled, preferably in a jet-mill.

Further Processing

Depending on the destination of the final product, the product from thedispersing step can be kept as a slurry, or spray-dried, or spray-driedand jet-milled as indicated in FIG. 1.

The following examples illustrate, but do not limit, the presentinvention.

EXAMPLES Example I

Lithium titanate made by spray drying according to U.S. Pat. Appln.Publication 2003/0017104 A1 and described above was further calcined inan oxidizing atmosphere at a temperature of 800° C. for 12 hours. Theproduct after calcination consisted of crystals of Li₄Ti₅O₁₂ of about400 to about 1000 nm in size. FIG. 4 is a scanning electron micrographof the product serving as starting material for the process of thepresent invention.

The product of the calcination step was further suspended in water andmilled with 0.4 to 0.6 mm zirconia grinding media for 8 hours. The BETsurface area of this product was 135 m²/g. This product was refired atconstant temperature for 3 hours. FIG. 5 shows electron micrographs ofthe product obtained after refiring at 900°, 650°, 500°, and 400° C.respectively. The particle size was about 100 nm at 400° C., andincreased to about 200, 500 and 1000 nm respectively at the highertemperatures. The micrographs show well-formed crystals with a narrowsize distribution.

Example II

Commercial lithium manganate, of particle size and shape shown in FIG.6, was slurried as a 40 weight % suspension in water and was milled in aPremier Mill bead mill for 20 h. The product of this operation wasdried, and then calcined in ceramic trays placed in a constanttemperature furnace for 3 hours. The results of calcinations atdifferent temperatures are given in the Table below: Calcinationtemperature Particle size (C.) (nm) uncalcined 15 400° 20 450° 30 500°50

Electron micrographs of each of the samples are shown in FIG. 7. Allcalcined crystals are well formed and show narrow size distributions.

While the invention has been described in conjunction with specificembodiments, it is to be understood that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, this inventionis intended to embrace all such alternatives, modifications, andvariations that fall within the spirit and scope of the appended claims.

1. A process for producing a nano-sized or sub-micron oxide of lithiumand a transition metal comprising: a. milling a coarse lithiumtransition-metal oxide; and, b. re-firing the lithium transition-metaloxide
 2. The process of claim 1 wherein the lithium transition-metaloxide is tetra lithium titanate spinel
 3. The process of claim 1 whereinthe lithium transition-metal oxide is LiCoO₂.
 4. The process of claim 1wherein the lithium transition-metal oxide is LiMn₂O₄.
 5. The process ofclaim 1 wherein the milling is accomplished by wet-milling in a beadmill.
 6. The process of claim 1 further comprising: a. dispersing theproduct resulting from re-firing to liberate crystallites; and, b.slurrying the crystallites to form a suspension.
 7. The process of claim1 further comprising milling the product resulting from re-firing toform a dispersed powder.
 8. The process of claim 6 further comprisingspray-drying the suspension.
 9. The process of claim 8 furthercomprising milling the spray-dried product to form a dispersed powder.10. A lithium-transition metal oxide with a particle size between 10 nmand 1000 nm and a standard deviation of no more than 20%.
 11. Alithium-transition metal oxide with a particle size between 10 nm and1000 nm and a standard deviation of no more than 20% made according tothe process of claim 1.